1. Field of the Invention
This invention relates to pistons for use in reciprocating combustion engines or reciprocating compressors or in hydraulic or pneumatic reciprocating motors and in particular to the reduction of friction between a co-operating cylinder wall or liner and the skirt of the piston and the reduction of the viscous oil losses in an oil film between these two co-operating surfaces.
2. Discussion of the Prior Art
The following is a more detailed description of a known standard piston, with reference to FIG. 1 of the accompanying drawings which is a side elevation of a known standard piston.
The known piston 11 has a ring belt 40, a crown 15, a skirt 12 and a gudgeon pin hole 13 extending through the piston normal to the piston axis. The skirt 12 is continuous and is usually slightly oval in cross-section. The skirt 12 of the piston performs no gas sealing function. Gas and oil sealing are provided by piston rings fitted into circumferential grooves in the crown of the piston.
In use, a connecting rod (not shown) is pivotally connected to the piston 11 by a gudgeon pin (not shown) extending through the hole 13. Since the connecting rod does not, in general, act along the piston axis, there is a varying lateral thrust force acting on the piston 11 which urges either a major or minor thrust portion of the skirt towards the associated cylinder during the working cycle. These thrust portions are on opposite sides of a plane including the axis of the gudgeon pin bore and the piston axis.
The whole circumferential extent of the skirt is thus not in contact with the associated cylinder or liner during reciprocation but only those thrust portions of the skirt which lie about the intersection of the skirt with a thrust plane including the piston axis and normal to the axis of the gudgeon pin hole. Since the lateral thrust force varies during reciprocation, the part of these thrust portions in contact with the cylinder or liner at any instant will also vary with time. In general, however, the part of a piston in instantaneous contact with the cylinder or liner will be within an area 14 commonly known as the bedding or contact area which is thus the envelope of all the instantaneous areas of contact. This bedding area may be larger on the major thrust portion of the piston, which contacts the cylinder or liner with the greater force during the expansion stroke, than on the opposite minor thrust portion which contacts the cylinder or liner with the greater force during the compression stroke.
The precise shape of the contact area will vary from piston to piston although it has been found that in general the contact area does not extend more than 30.degree. in either side of the intersection of the thrust plane with the skirt. It has also been found that for a piston having a nominal diameter D and a skirt length of .pi.2D/3, the contact area is approximately .pi.D.sup.2 /9. Such a piston is hereinafter referred to as a `standard piston` and such a contact area as `a standard contact area`.
The sliding contact between the skirt and the cylinder or liner is lubricated by an oil film. The frictional force (F) between the piston and the cylinder or liner as a result of this contact is given by the following relationship: ##EQU1## where W=load
.eta.=lubricant viscosity PA1 U=sliding velocity PA1 A=contact area
Contact between the thrust portions of the skirt and the associated cylinder or liner causes a high proportion of the friction losses, for example, 20% of the total engine friction losses (corresponding to about 8% of the mechanical output), which in turn reduces fuel economy. Thus a reduction in friction of, say, 25% between the skirt and the cylinder or cylinder wall can provide an additional mechanical output of about 2%, thereby in this example reducing the fuel required for a given duty by just under 2%.
This relationship implies that for a given load, viscosity and velocity, the frictional force can be reduced by reducing the area of contact. This is clearly desirable since, as mentioned above, some of the engine power is consumed in overcoming this friction.
However, a reduction in contact area also reduces the thickness of the oil film between the skirt and the cylinder or liner. Only a limited reduction in this thickness can be tolerated because when the thickness of the oil film is less than the height of asperities on the surface of the skirt and the cylinder or liner there will be metal-to-metal contact and the frictional forces will rapidly rise.